How To Make Html/php Forms Read New Lines?

I've been wondering this for a while, and after searching, found no answer.

With a form for HTML/PHP, like the message posting thing here, how do you make the code read new lines and save them as <br>s, rather than as one long chunk of text without new lines? I can't figure it out...

My forms, as they are, would ignore all the new lines in this and make this into one very long line.